The International Space Station (ISS) Active Thermal Control System uses a single phase liquid ammonia system to collect and reject waste heat from the various space station systems. The expected cold environments in which the Heat Rejection Subsystem (HRS) radiators of the heat rejection system are to operate fall as low as -102.8 °C (-153 °F). Because the ammonia working fluid freezes at -77.7 °C (-108 °F) and since the environment temperatures are to remain below this level for 30 minutes per orbit, design approaches have been identified, implemented, and tested to ensure that the ISS Active Thermal Control System radiators will perform under these environments.There are several items of concern in a freeze-tolerant design. The flow tubes imbedded in the panel, from which heat is rejected, must be designed to tolerate potentially high pressure during a thaw. The supply and return manifold tubing must be designed to prevent ammonia from freezing within them. The panel design must guarantee that a sufficient number of flow tubes remain open during freezing conditions. And, the concepts adopted to address these issues must not significantly impair the hot environment performance.As part of McDonnell Douglas Aerospace contract #919S6003 under which the HRS radiators are being developed, a qualification level radiator panel was built, instrumented and tested in a thermal vacuum chamber to demonstrate these concepts. Results of the test are documented within this paper.